1. Field of the Invention
This invention concerns a heat sensitive recording sheet which is suitable to high density and high speed recording and excellent in the storage stability of images.
2. Prior Art
Generally, heat sensitive recording sheet is prepared by usually grinding to disperse usually colorless or pale colored basic dye and a developer such as of phenolic substance into fine particles respectively, mixing them, adding binder, filler, sensitivity improver, lubricant and like other auxiliary agents thereto and coating the thus obtained coating liquid to paper, film or like other support, for obtaining color-forming recording through instantaneous chemical reaction upon heating. In this case, colors of various hues can be formed by selecting the kind of colorless dyes.
While a lot of substances are described as the developer for heat sensitive recording sheet in various literatures including Japanese Patent Publication No. 14039/1970, 4,4'-isopropylidene diphenol (bisphenol A) has most widely been utilized so far in view of the stability of the quality, cost and availability. However, bisphenol A has a defect in that the thermal color forming temperature is high and can not be adapted sufficiently to the recording with a minor heat energy and that troubles such as sticking have been liable to occur.
In recent years, higher speed recording and improvement in the image quality have been demanded for the heat sensitive recording sheet, and in compliance therewith, the present applicant has already proposed a heat sensitive recording sheet using p-hydroxy benzyl benzoate as the developer, which is combined with a fluoran type dye (refer to Japanese Patent Laid-Open No. 144193/1981), and made it clear that in improvement in the sensitivity with excellent dynamic color-forming property can be attained with ease. However, heat sensitive recording p-hydroxy benzyl benzoate as the developer has a problem in that the density of images formed by the heat application is reduced with the elapse of time, or p-hydroxy benzyl benzoate melted once by the application of heat causes recrystallization with the elapse of time to produce white powder on the surface of the images.